Physical activity and its measurement have important implications for health and wellbeing. Low levels of physical activity are associated with poor health, such as cardiovascular disease and obesity. Additionally, physical activity measurement gives a quantitative measure of an individual's community mobility. This is important for subjects with an impairment resulting in reduced mobility, for example patients who have had a stroke, or who exhibit behavioural changes, such as bipolar disorder. Stepping is the most important of the primary activities in relation to physical activity. The energy cost of purposeful walking is around four times resting values and the energy expenditure of running can be more than ten times resting values.
Pedometers are the simplest and cheapest devices available for quantifying stepping activity. These provide a cumulative step count that the user can periodically reset. Typically, pedometers are designed to measure purposeful walking and tend to be inaccurate at counting slow steps or fast stepping, e.g. running. Typical pedometers do not measure the intensity of stepping, often referred to as cadence, and provide only a total count of steps taken over a time period and some simple calculations based on this total. There is a range of monitors available which provide a continuous record of the steps taken. Devices that provide this facility include the Actigraph (Actigraph LLC, Florida, USA) and the Stepwatch (CYMA Corporation, Seattle, USA). These record the number of steps taken in a defined period of time, typically one minute.
A problem with devices that record only the number of steps taken is that they cannot accurately reflect the intensity of stepping. For example, if 120 steps were taken in a minute then the cadence is 120 steps per minute. If only 20 steps were taken in the minute then this would be represented as 20 steps per minute. However, if the 20 steps were taken in the minute but only 10 seconds was actually spent stepping then the true cadence would be 20 steps in 10 seconds, which is equivalent to 120 steps per minute. This represents an error in estimating the intensity of the stepping activity of a factor of six and misclassifies a high physical activity period as a low level period.
As well as simple step counters, there are some other, more complex devices available to provide a measure of the variation in stepping intensity over extended periods. For example, the activPAL (PAL Technologies Ltd, Glasgow, UK) and the AMP311 (Dynastream Innovations Inc, Alberta, Canada). These use sophisticated algorithms to detect and record parameters associated with each individual step. This allows a detailed picture of stepping activity to be constructed from the recorded data. Having a detailed record of the pattern of stepping activity is important as the energy cost of a step is not fixed and varies with cadence. Broadly, the energy cost of walking a mile is the same as running a mile, i.e. the energy cost per unit of distance traveled is similar. However, as stepping rate increases stride length also increases. Hence, at higher stepping rates each unit of distance is covered in less time and the physical effort, i.e. the energy cost, of the stepping activity is increased.
Whilst some of the more complex devices provide useful information, a problem with these is that they do not provide a measure of true cadence and the power requirements are high. This is because they need a relatively large on-board memory and processing capability. This means that these devices are either unsuitable for long term use, or would have to be made relatively large to accommodate a big enough battery. Neither of these scenarios is ideal where the monitors are to be used by patients who have long-term mobility problems and where information on their levels of activity could provide valuable information. For example, lower limb amputees use a prosthetic leg and they typically undergo an annual assessment. This leg is a medical device and its performance is governed by international standards. By monitoring prosthetic leg function and use over the extended period between clinic appointments, valuable information on device performance and patient wellbeing can be generated. However, current devices are not suitable for this type of use, and so there is currently no opportunity to obtain such valuable information over extended periods.